The xylenes, such as para-xylene, meta-xylene and ortho-xylene, can be important intermediates that find wide and varied application in chemical syntheses. Generally, para-xylene upon oxidation yields terephthalic acid that is used in the manufacture of synthetic textile fibers and resins. Meta-xylene can be used in the manufacture of plasticizers, azo dyes, and wood preservers. Generally, ortho-xylene is a feedstock for phthalic anhydride production.
Xylene isomers from catalytic reforming or other sources generally do not match demand proportions as chemical intermediates, and further comprise ethylbenzene, which can be difficult to separate or to convert. Typically, para-xylene is a major chemical intermediate with significant demand, but amounts to only 20-25% of a typical C8 aromatic stream. Adjustment of an isomer ratio to demand can be effected by combining xylene-isomer recovery, such as adsorption for para-xylene recovery, with isomerization to yield an additional quantity of the desired isomer. Typically, isomerization converts a non-equilibrium mixture of the xylene isomers that is lean in the desired xylene isomer to a mixture approaching equilibrium concentrations. This isomerization effluent may be recycled to a xylene-isomer recovery unit. It is also desirable to convert ethylbenzene to one or more xylenes while minimizing xylene loss. Although not wanting to be bound by theory, it is believed that one or more C8 naphthenes can be created as intermediates for converting ethylbenzene into one or more xylenes. Moreover, other desired aromatic products, such as benzene, can be produced from such processes.
Various catalysts and processes have been developed to effect xylene isomerization. In one such system, isomerization can include separate reactors having different functions. Particularly, one reactor having a first isomerization catalyst can perform xylene isomerization with low ethylbenzene conversion, while the other reactor having a second isomerization catalyst may perform ethylbenzene conversion with low xylene isomerization. If the ethylbenzene reactor can selectively convert ethylbenzene into one of the xylene isomers, typically para-xylene, then above-equilibrium levels of the preferred isomer can be obtained. Depending on the isomerization catalyst, the ethylbenzene may also be converted to xylenes, or may simply be dealkylated. In this way, the desired product yield is maximized by converting the undesired components. Yield is greatest when undesired products can be minimized and ethylbenzene conversion can be maximized.
One way to reduce loss of cyclic hydrocarbons having eight carbon atoms (hereinafter may be abbreviated as “C8 ring loss” or “C8RL”) is to operate in a liquid phase. In the absence of hydrogen, saturation and cracking reactions may be essentially eliminated.
Typically, C8 naphthenes are not intermediates for xylene isomerization, although the C8 naphthenes may be included in the feed to the xylene isomerization zone. Alternatively, if the ethylbenzene level is very low or zero, only the xylene isomerization may be required.
Thus, it would be desirable to improve the operation of the liquid-phase xylene isomerization by altering the feed to reduce C8RL while increasing isomerization activity.